The invention relates to a method for the regeneration of iron-loaded denox catalysts.
During the production of current using fossil fuels exhaust gases are obligatorily produced that contain in particular nitrogen oxides and sulfur dioxides in addition to fine dust as environmentally harmful compounds. The exhaust gases must therefore be cleaned from these compounds to the extent possible before they can be put into the environment, that is, in other words a desulphurization as well as a denitration and a removal of fine dust by filters are necessary. The desulphurization is carried out according to different methods in which basically the SO2 produced in the combustion is oxidized to SO3, then absorbed in alkaline solution and finally removed usually in the form of gypsum. The denitration is carried out parallel to the above during which nitrogen monoxide with ammonia and atmospheric oxygen is converted to elementary nitrogen and water and nitrogen dioxide also reacts with ammonia and atmospheric oxygen to elementary nitrogen and water. This reaction requires catalyst designated as so-called denox catalysts. These are catalysts with a glass fiber body or catalysts with different shapes, preferably honeycomb-shaped or plate-shaped, based on titanium dioxide and containing the oxides of various transition metals such a vanadium, molybdenum and tungsten as active components.
The effectiveness of such catalysts decreases after an operating time of, e.g., on the order of 30,000 hours as a function of which fuel is used in the power plant, which is conditioned on the one hand by fly ash settling in and/or clogging the catalyst passages and on the other hand by the formation of blocking layers by the ammonium sulfate formed during the denitration and in addition by a toxification of the active centers by elements or compounds such as, e.g., arsenic, phosphorus or metals contained in the exhaust gas.
A special problem is posed by the reduction of performance by the undesirable elevation of the SO2/SO3 in the denitration range by iron compounds. When coal is used as fuel it must be taken into consideration that coal can contain a not insignificant amount of mineral components, depending on its age and origin, and the iron content can be in a range of usually 5 to 7 or 8 wt. % relative to the total amount of the mineral components.
Iron compounds not only settle permanently mechanically in the catalyst on the surfaces but also enter into chemical reactions with the catalytic components, thus resulting in a diminution of the catalytic performance during denitration.
The removal of metals from denox catalysts while retaining structure and activity is described, e.g., in DE 43 00 933 in which two different gaseous phases are used. However, this method is not suitable for removing other pollutants from the catalyst. All previously known methods for the regeneration of denox catalysts that operate with reaction liquids such as, e.g., EP 0 910 472; U.S. Pat. No. 6,241,826; DE 198 05 295; DE 43 00 933; EP 0 472 853; U.S. Pat. No. 4,914,256 cannot specifically remove iron. That is, in other words, there was previously no possibility of treating catalytic disturbances in the form of an elevation of the SO2/SO3 conversion rate due to iron.